Although MRI testing is becoming increasingly important in diagnostic medicine, it is contraindicated for some patients. Such contraindication may result from an active implanted medical device.
Besides possible heating effects of implants, in particular for small conductive structures (not addressed here), there are problems with erroneous identifications of events in the heart, including but not limited to ventricular fibrillation (VF), i.e., rapid disturbances in cardiac rhythm, and the high static magnetic fields and resulting magnetization of electrical components, in particular for active implants, including but not limited to implanted cardiac defibrillators/cardioverters, referred to below as ICD. However, other battery-powered devices may also be functionally impaired by electromagnetic fields, and cannot be used in environments having increased electromagnetic loads. The following prior art primarily addresses the problems with implanted medical devices (IMD).
U.S. Pat. No. 6,522,920 describes a system for protecting the inductively actuated high-voltage switches of an ICD in the delivery of shocks in a magnetic field. For this purpose monitoring is performed to determine whether sufficient gate voltage is present at the time of delivery of the shock. The monitoring is assessed by an evaluation of the secondary voltage of the inductive drivers.
For detection of a magnetic field it is known from US Patent Application Publication 2008/0071168 discloses providing an impedance measuring unit for detection of a magnetic field and an RLC (Resistor-Inductor-Capacitor) element, and to perform the detection by determining the inductance in the RLC component of the implanted device. This requires an additional or modified impedance measuring unit.
In addition, U.S. Pat. No. 7,509,167 describes the detection of a magnetic field by measuring the control times for actuating the high-voltage transformer for the primary side, or alternatively, measuring peak currents during a charging cycle. A disadvantage of the described system is that the control times must be measured very quickly, because they are in the μs to ns range; i.e., it is difficult to perform a direct current measurement due to the necessary measurement resistance, which would prolong the charging times for the high-voltage capacitor.
What is needed is a way is to provide magnetic field detection, in particular MRI detection, so that it is easy to implement.